Good old memorisation of entire essays vs techniques/quotes debate. I can't be bothered reading the above responses, but here are my two cents as a student who graduated 1st in Advanced and Extension from a selective school with a 96 and 49 after memorising all my essays.
The fact of the matter is that to achieve 18+/20 and 13.5+/15 your essay needs to be perfectly relevant, sophisticated, coherent, and synthesised, which you are expected to be able to exercise on the exam day, unfortunately making it a gifted talent that you either have, or don't have. Memorising a pre-written generic essay that has been proof-read meticulously by not just yourself, but teachers, tutors, and high ranking peers as well, grants you the ability to have sentence structures that are already sophisticated, coherent, and synthesised , meaning that you are more or less on par with students that have the talent of writing. Of course, that leaves the significant issue of being relevant, which is where many students fail in regards to memorising essays. What this means is that your essay needs to be readily adaptable, which actually isn't terribly difficult. The concept of "curve-ball questions" has always annoyed me because in my honest opinion, there are no such things as "curve-ball questions". It is a term students apply to a question they're limited knowledge/understanding of the texts, modules, syllabus, and concepts is unable to answer. There has not been a single question in the past papers that I have seen which I could not answer even without having read Romulus, My Father and Frankenstein (I got half way through RMF and didn't bother opening Frankenstein at all) because I took the time to read the syllabus and understand it fully. Admittedly, some of my internal questions have been just a little more challenging than the usual, e.g. something along the lines of "Write a set of diary entries that express your understanding of Hamlet as the play progressed and your understanding, including its influences, evolved." (Obviously the question is asking for something simple, but the set of diary entries in terms of how to structure each entry was a little more challenging. However, I was the only student to receive 20/20 for this question too, with teachers saying my understanding of the play was clearly excellently developed and insightful.) Where I am going with this is that for relevance to be achieved a sophisticated understanding of the texts, modules, syllabus points, and concepts must be acquired by the student. This is where having extra quotes and techniques come in handy, but what is also interesting is that most of the time, the pre-written essay is drafted over and over again, with points, quotes and techniques changing each draft. This means that you actually have a bank of extra quotes and their respective techniques already, just not written into the specific pre-written essay, but if your essay is indeed adaptable then alternating shouldn't be too difficult, and of course, as your essay already has structures for being sophisticated, coherent, and synthesised, you're easily in line for a high A range response. Other benefits of memorising your essay involve feeling more prepared and therefore less stressed on the exam day, and before people say "what if you have a mental blank", well that's a risk everyone memorising has already considered and yet they have still chosen to memorise, and I could just as easily say "well what if you forget your quotes and techniques" or "what if you don't know how to structure your sentences coherently".
Don't get me wrong, I'm a strong believer that memorising quotes and techniques is the way English is supposed to be studied, but that means only the next J. R Tolkein, Gabriel Garcia Marquez, Tim Winton, or Jane Austen can be a high band 6 achiever. Memorising essays helps to put you on par with students like these, and at the very least you have a response you can sort of limp through adapting in the exam and get a 14+ (remembering the state average for most modules sit around 12/20 and 9/15) on harder questions.
Quite frankly a student who memorises and only gets a 16 is someone who hasn't done it right because they lack an understanding of the module and text. There is more to memorising essays than word-for-word regurgitation, contrary to what many criticisers of memorisation seem to believe. It will give you a competitive edge, but that's still only if you're a competent English student.